This invention relates to electroplating metal layers onto substrates and electropolishing metal layers on substrates. More specifically, it relates to apparatus for controlling the composition, flow, and potential distribution of electrolyte while electroplating or electropolishing a silicon wafer.
Damascene processing is a method for forming metal lines on integrated circuits. It is a significant departure from traditional methods that require blanket deposition and subsequent patterning of aluminum. In comparison to such traditional processes, Damascene processing requires fewer processing steps and offers a higher yield. In Damascene processes, copper is a favored over aluminum because of its higher conductivity and resistance to electromigration.
In a typical Damascene process, copper is deposited in at least two steps. First, the process deposits a very thin layer of the metal by physical vapor deposition (PVD). Then, the process forms a thicker copper electrofill layer by electroplating. The PVD process is typically sputtering. One example of a commercially successful apparatus that electroplates copper onto wafer active surface is the SABRE(trademark) electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. patent application Ser. No. 08/969,984, xe2x80x9cCLAMSHELL APPARATUS FOR ELECTROCHEMICALLY TREATING SEMICONDUCTOR WAFERSxe2x80x9d naming E. Patton et al. as inventors, now U.S. Pat. No. 6,156,167 and U.S. application Ser. No. 08/970,120, now U.S. Pat. No. 6,159,354 both filed Nov. 13, 1997, which are herein incorporated by reference in their entirety and for all purposes.
Electroplated copper should fill Damascene trenches from the xe2x80x9cbottom-up.xe2x80x9d If instead the copper plates on the top and side-walls of the Damascene trenches, voids form in the conductive lines, reducing conductivity and causing the integrated circuit to be unusable. The plating electrolyte (often referred to as the xe2x80x9cplating bathxe2x80x9d) composition helps control the conformation of electroplated copper. Certain organic additives known as xe2x80x9cacceleratorsxe2x80x9d or xe2x80x9cbrightenersxe2x80x9d significantly improve the copper feature filling when added to the electrolyte. In fact, a significant technical challenge in plating copper on integrated circuits involves maintaining stable additives in the electrolyte. If the additives degrade with use, one cannot achieve consistent bottom up plating. A discussion of bath degradation and maintenance strategies can be found in xe2x80x9cUse of On-Line Chemical Analysis for Copper Electrodeposition,xe2x80x9d R. J. Contolini, J. D. Reid, S. T. Mayer, E. K. Broadbent, and R. L. Jackson, Advanced Metallization Conference, 1999, Sep. 28-30, 1999, Orlando, Fla., Paper #27. In general, a controlled composition of a plating bath is essential to maintain good bottom-up electroplating, uniformity and other desirable plating characteristics. While certain organic additives promote bottom-up plating, other compounds interfere with such plating. Some of the interfering compounds are decomposition products of the desirable accelerators. It has been found, for example, that poor plating is often associated with decomposition of accelerators. Many plating baths contain accelerators such as dimercaptopropane sulfonic acid (SPS) or N-dimethyldithiocarbamic acid (DPS). These can breakdown to their monomers (e.g., mercaptopropane sulfonic acid (MPS)). Small amounts of MPS in an SPS or DPS bath can substantially degrade bottom-up filling.
Additive degradation may be mitigated by periodically dumping an old bath and adding a fresh bath (sometimes termed a xe2x80x9cbleed and feedxe2x80x9d or a xe2x80x9cbath replenishmentxe2x80x9d procedure). U.S. Pat. No. 5,352,350 issued to Andricos et. al. describes an embodiment of this approach. In theory, this approach maintains the concentration of xe2x80x9cpoisonsxe2x80x9d (e.g., MPS or other breakdown product) at an acceptable steady state value. Unfortunately, it produces a substantial volume of waste and requires a continuous detailed bath analysis using maintenance metrology. Waste generation is environmentally problematic and requires costly treatment. Further, as wafer diameters increase (as they will continue to do), the amount of required dumping increases.
In another approach, the plating apparatus may include an adsorption column (e.g., an activated carbon fluidized bed) to remove poisons. Generally, such beds lack the specificity to remove only the unwanted poisons. Thus, this approach typically strips all additives from the plating bath to create a xe2x80x9cvirginxe2x80x9d solution. This solution is then reintroduced to the main bath together with appropriate levels of fresh additive. Unfortunately, this approach is often uneconomical because it requires (i) processing of large volumes of plating bath, (ii) large quantities of fresh accelerators and other expensive additives, and (iii) a large carbon filter, which need to be replaced frequently.
Most organic additive breakdown processes occur at the anode surface. To reduce breakdown, plating systems may employ copper anodes containing 0.02 to 0.04% phosphorus. Such anodes form a surface film with better tenacity and less particle generation than non-phosphourus containing anodes and also act as a protective diffusion barrier for brighteners (see Modern Electroplating, Frederick A. Lowenheim, editor, Third edition, pg 192). Still, the film has a particulate morphology and accumulates breakdown products. Also, it has been found that the bath plating quality (as evidenced by copper layer conformation and defects) is strongly sensitive to disturbances in the anode film caused by stirring and other mechanical perturbations commonly employed in modem electroplating apparatus.
There are generally two classes of anodes that are used in metal plating: consumable (also referred to as active) anodes, and non-consumable (also referred to as xe2x80x9cdimensionally stablexe2x80x9d and non-reactive) anodes. The reactions of the active anode for plating copper are simple and balanced (no overall depletion or generation of new species). Copper ion in the solution are reduced at the cathode and removed from the electrolyte, simultaneously as copper is oxidized at the anode and copper ions added to the electrolyte. In contrast, the reactions in a non-consumable system are unbalanced. The two reactions are:
H2Oxe2x86x92xc2xdO2+2H++2exe2x88x92(anode)
Cu+2+2exe2x88x92xe2x86x92Cu (cathode).
U.S. Pat. No. 4,469,564 issued to Okinaka et al. describes a copper electroplating system in which the non-consumable anode is surrounded by a cation exchange membrane. The membrane prevents passage of organic additives and anions, and thereby prevents the organic additives from contacting the non-consumable anode, while allowing passage of positive ions (generally hydrogen) to pass cationic current. When the membrane is present, the anode chamber will accumulate hydrogen ion. Accordingly, xe2x80x9cthe feature is especially advantageous for copper electroplating processes using non-consumable electrodes because of the high consumption of additives and that the copper can be added to the cathode side of the membrane so that acid copper ions need not pass through the membrane.xe2x80x9d Unfortunately, the resistance to ion mass transport across such membranes is great.
U.S. Pat. No. 5,162,079 issued to Brown describes an electroplating system in which the non-consumable anode is enclosed in a compartment having a nonporous anion or cation exchange membrane with a means of flushing the anode compartment to maintain the acid concentration there
What is needed therefore is improved electroplating technology that reduces the rate at which additives break down, minimizes power consumption, improves the bath stability/longevity, and minimizes chemical waste generation.
The present invention overcomes the above difficulties caused by anode mediated degradation of electrolyte additives by separating the electrolyte into a portion associated with the anode and a portion associated with the cathode (anolyte and catholyte, respectively). The separation is preferably accomplished by interposing a microporous chemical transport barrier (sometimes referred to as a diffusion barrier) between the anode and cathode. The transport barrier should limit the chemical transport (via diffusion and/or convection) of most species but allow migration of anion and cation species (and hence passage of current) during application of electric fields associated with electroplating. In other words, the transport barrier should limit the free cross-mixing of anolyte and catholyte.
A general advantage of separate anolyte and catholyte chambers is the ability to separately use materials having vastly different physical and chemical properties in the two chambers. Examples of such properties include viscosity, metal ion concentration, water concentration, conductivity, and, importantly, organic additive concentration. Some conditions are better suited for the anode and others for the cathode. Generally, in electroplating baths, any poison forming organic additive should be kept out of the anode chamber.
The designs of this invention have additional benefits. Particles that are often generated at the copper anode are prevented from passing into the cathode (wafer) chamber area and thereby causing a defect in the part. In a conventional design, the anode and cathode are exposed to the same electrolyte. Initially the anode contains no protective film and therefore the rate of additive consumption is high. It is often required therefore to condition the anode and plating bath by passing a substantial amount of current through the cell prior to plating product wafers to establish an equilibrium condition. Several bleed-and-feed bath additions may also be required to reach a steady state condition. In the design of this invention, such start up processing is not necessary. Furthermore, in most conventional designs, when plating is not performed in a cell, the anode is still immersed in the bath electrolyte. Under these conditions, the anode protective film slowly degrades. (It is believed to slowly dissolve or be oxidized by dissolved oxygen from air in the electrolyte.) The consumption of plating bath additive can be dependent on a number of anode related conditions such as 1) the charge passed through the cell 2) the time since cell use, 3) the flow in the anode compartment, and other physical changes. In the design of this invention, these dependencies are removed. One aspect of this invention provides an apparatus for electroplating copper onto a substrate. The apparatus may be characterized by the following features: (a) a cathode electrical connection that can connect to the substrate and apply a potential allowing the substrate to become a cathode; (b) an anode electrical connection that can connect to an anode and apply an anodic potential to the anode; and (c) a porous transport barrier defining an anode chamber and a cathode chamber. The porous transport barrier enables migration of ionic species, including copper ions, across the transport barrier while substantially blocking diffusion or mixing (i.e. transport across the barrier) of solvent or solutes between the anolyte and catholyte, thereby preventing non-ionic organic bath additives from crossing the transport barrier. The ionic species are driven across the barrier by migration (movement in response to the imposed electric field) but the neutral species do not transverse the transport barrier. Generally, the anode chamber contains an anolyte and the cathode chamber contains a catholyte. The transport barrier maintains different chemical compositions for the anolyte and the catholyte.
In one embodiment, the anolyte includes one or more copper salts (e.g., copper sulfate) dissolved in water. It is substantially devoid of organic species, particularly accelerators. The electrolyte also can contain an acid. Typical formulations for the anolyte have between about 10 and 50 gm/l copper (as Cu+2), and between about 0 and 200 gm/l H2SO4. More preferably, the anolyte concentration of copper is between about 15 and 40 gm/l and the concentration of acid is between about 0 and 180 gm/l H2SO4. Examples of two preferred formulations of electrolyte are (1) about 40 gm/l Cu+2 and at most about 10 g/L H2SO4 (referred to as a low acid formulation) and (2) about 18 g/L Cu+2 and about 180 g/L H2SO4 (referred to as a high acid formulation). Generally, the catholyte contains a substantially greater concentration of the non-ionic organic plating additives than the anolyte.
In one preferred embodiment, the apparatus includes an anolyte storage reservoir or source connected to the anode chamber to provide anolyte to the anode chamber. The apparatus in this embodiment may also include a conduit between the anode reservoir/source and the cathode chamber allowing periodic delivery of electrolyte from the anode reservoir to the cathode chamber. The apparatus may also include a catholyte storage reservoir connected to the cathode chamber to provide catholyte to the cathode chamber.
In one embodiment, the apparatus further includes a conduit allowing removal of electrolyte from the cathode chamber. This electrolyte may be provided to an electrolyte treatment system that treats the electrolyte for reintroduction to the electroplating apparatus. Preferably, the electrolyte treatment system includes an activated carbon absorbing medium. The treated electrolyte may have its additives substantially removed so that it can be introduced into the anode chamber.
Various materials may be used in the transport barrier. Examples include porous glasses, porous ceramics, silica aerogels, organic aerogels, porous polymeric materials, and filter membranes. In a preferred embodiment, the transport barrier is made from a sintered polyethylene or a sintered polypropylene.In a specific embodiment, the apparatus includes a carbon filter layer that is substantially coextensive with the transport barrier. The carbon filter layer can filter non-ionic organic bath additives from a catholyte that manage to pass through the transport barrier toward the anode chamber. In an especially preferred embodiment, the transport barrier comprises a three-layer membrane including a first layer of porous material sandwiched between two additional layers of porous material. In this embodiment, the first layer is substantially thinner than the two additional layers.
In some embodiments, the anode chamber and the transport barrier are designed or configured to allow a limited flow of anolyte from the anode chamber into the cathode reservoir or chamber. This is permitted while substantially preventing catholyte from flowing from the cathode chamber into the anode chamber. Such systems may allow concentrated anolyte to exit directly to the cathode chamber and thereby maintain desired concentrations anolyte and catholyte with minimal external apparatus. The flow of anolyte to cathode chamber also allows xe2x80x9cflushingxe2x80x9d of any inadvertent or otherwise unpreventable transport of additive that are transported to the anolyte chamber.
Another aspect of the invention provides a method of electroplating copper onto a substrate in a manner reducing the likelihood of generating electrolyte species that inhibit bottom-up plating on the substrate. The method may be characterized by the following sequence: (a) providing anolyte in an anode chamber having an anode and being separated from a cathode chamber by a transport barrier as described above; (b) providing catholyte to the cathode chamber containing the substrate; and (c) applying a potential difference between the substrate and the anode to allow the substrate to become a cathode and plate copper metal onto the substrate without allowing the concentration of plating additives in the anolyte to substantially increase. Other features of the method may include (i) providing anolyte to the anode chamber from an anolyte storage reservoir and/or (ii) transferring anolyte to the cathode chamber.
In many embodiments, the method will also require removing catholyte from the cathode chamber and routing the removed catholyte to a catholyte reservoir (which may be a central plating bath for multiple plating cells, for example). The catholyte from a catholyte storage reservoir may be cycled back to the cathode chamber to maintain good convection.
In some embodiments, catholyte will be treated to convert it to anolyte. Such method may be characterized as follows: (a) removing catholyte from the cathode chamber; (b) treating the catholyte to reduce the concentration of organic additives in the catholyte to produce an anolyte that is substantially free of additives; and (c) introducing the treated anolyte produced at (b) to the anode chamber. In a preferred embodiment, the catholyte is treated by passing it through an activated carbon absorbing medium or a reverse osmosis separation apparatus, or a combination of these.
Note that separate anode and cathode chambers of this invention can also be used in electropolishing, which has its own set of separate requirements for the anolyte and catholyte. Thus, another embodiment of this invention provides an apparatus for electropolishing copper on a substrate. As in the electroplating embodiment, the apparatus includes a transport barrier defining an anode chamber and a cathode chamber. In a preferred example, the transport barrier maintains the viscosity of the catholyte at a substantially lower value than the viscosity of the anolyte.
Another aspect of the invention provides copper electroplating apparatus that may be characterized by the following features: (a) separate anode and cathode chambers ionically connected to one another; (b) an anolyte flow loop that circulates anolyte into, out of, and through the anode chamber; and (c) a catholyte flow loop that circulates catholyte into, out of, and through the cathode chamber. The apparatus substantially prevents anolyte and catholyte from mixing and thereby maintains different compositions of anolyte and catholyte. Preferably, the separate anode and cathode chambers are maintained separate by a transport barrier as described above. This prevents free mixing of the anolyte and catholyte and thereby allows the compositions of anolyte and catholyte to remain different in their respective flow loops.
A somewhat different aspect of the invention pertains to another method of electroplating copper onto a substrate, in a manner reducing the likelihood of generating electrolyte species that inhibit bottom-up plating on the substrate. This method may be characterized by the following sequence: (a) cycling anolyte through an anolyte flow loop including an anode chamber having an anode; (b) cycling catholyte through a catholyte flow loop including a cathode chamber having a cathode onto which copper is electroplated, wherein the anode chamber and cathode chamber are separate but ionically connected chambers; and (c) passing current through the cathode, catholyte, anolyte, and anode to allow electroplating of copper onto the cathode. Preferably, (a), (b), and (c) are performed concurrently. Various aspects of this method may be performed and implemented as described above.